Dewaxing oils with periodic changes in solvent composition



March 13, 1956 J. G, PRATT, JR., ETA]; 2,738,306

DEWAXING OILS WITH PERIODIC CHANGES IN SOLVENT COMPOSITION Filed June 15, 1953 22 a as \c \z B I w A A r A STORAGE k W W STORAGE I M k is SECOND H251 FLASH zone FLASH ZONE 7.\ a 1 w \o DEWAXED O\L V PRODUCT OIL- SOLVENT MUCTURE soLunoN REFRIGERATION AND SEPARATED WAX WAX SEPARAHON ZONE 4 USULLATE ZE'HDUE lnvzni'ors: John Goibraifh PraTT Jr.

John Nicholas Umbach Jr.

B 4 hair Afforncg United States Patent "O DEWAXING OILS WITH PERIODIC CHANGES IN SOLVENT COMPOSITION John G. Pratt, Jr., and John N. Limbach, Jr., Houston,

Tex., assignors to Shell Development Company, Emeryville, Calif., a corporation of Delaware Application June 15, 1953, Serial No. 361,792

4 Claims. (Cl. 196-18) This invention relates to a method for the removal of wax from mineral oil, and it pertains particularly to solvent dewaxing processes wherein it is desirable to periodically change the composition of the solvent.

Solvent dewaxing of mineral oils, particularly of waxy lubricating oil distillate fractions and also of waxy short residues, is well-known and widely practiced on a commercial scale. It is the general practice in solvent dewaxing to utilize as the so-called solvent or diluent mixture of two or more liquid substances. These components, usually two or three, are blended in such proportion that the wax will be precipitated as completely as possible at the desired dewaxing temperature, the desired dewaxing temperature or temperatures depending on the desired properties of the separated wax, or the desired properties of the recovered oil, or both. In such a diluent or solvent mixture the component or components of higher solvent power may be called the solvent and the component or components of lower solvent power may be called the anti-solvent. The term solvent mixture as used hereinafter will refer to a mixture of the solvent and the antisolvent. Usually it is desired to use as much anti-solvent as can be tolerated without causing the separation of a separate liquid oil phase at the dewaxing temperature. The result is that when the composition is so adjusted that the oil solution is on the verge of phase separation the solution will have the minimum solvent power for wax and still adequate solvent power for the oil components.

In a number of present commercial solvent dewaxing operations, the paratfinic or mixed base crude lubricating oil stock is fractionated under reduced pressure to produce two or more, generally three, lubricating oil distillate fractions, leaving aso-called waxy short residue, from which the so-called bright stock lubricating oil is recovered. The distillate fractions may or may not be solvent extracted and the short residue is deasphalted, as by propane deasphalting, and may or may not be solvent extracted, and the distillate and bright stock fractions are then subjected to solvent dewaxing. It is the usual practice to carry out the dewaxing of these materials in socalled blocked-out operations, wherein the same plant is used at selected times for dewaxing a collected amount of a given one of the fractions, since it is. more economical to build a larger dewaxing unit for handling all of the lube stock in blocked-out operations than to provide separate smaller dewaxing units, one for each of the lube fractions to be dewaxed. In such operations, part or all of the different waxy lubricating oil fractions usually are dewaxed separately at about the same dewaxing (filtering, centrifuging or settling) temperature, yielding dewaxed oils'having aboutthe same so-called pour points. It is necessary to separate the wax from the wax-solvent-oil slurry at a temperature somewhat below the desired pour point temperature of the oil to be recovered therefrom. This temperature dififerential .(pour point temperature minus dewaxing temperature) depends on the nature of the" waxy lubricating oil fraction, the particular solvent blend and the manner in which the wax is precipitated 2,738,306 Patented Mar. 13, 1956 be as low as possible, thereby resulting in a high capacity of the equipment and in a sharp separation of wax from solution. Additionally, refrigeration costs will thereby be reduced.

However, when the same stream or mass of solvent mixture or blend is alternately used in dewaxing distillate fractions and in dewaxing a residual stock or relatively higher molecular weight distillate fraction or fractions, if the dewaxing solvent mixture composition is adjusted to the optimum composition for the dewaxing of the distillate fraction or fractions, then the same solvent mixture has too high a proportion of anti-solvent to be tolerated in dewaxing of the heavier fraction without separation of a separate oil phase. On the other hand, if the solvent mixture composition is adjusted for optimum operation when dewaxing the heavier oil fraction (e. g., bright stock or a high molecular weight distillate fraction), then the same solvent mixture will require an excessive temperature differential between pour point temperature of the dewaxed oil and the dewaxing temperature, when it is used to dewax the lower molecular weight distillate fraction(s) Thus, it is desirable, or sometimes even necessary, to change the composition of the solvent mixture periodically for the processing of the different charge stocks.

Similarly, when only one type of charge stock is to be processed, and that type continuously, it is often desirable to produce dewaxed oil of different pour points at diiferent times. It is well known in this case too that if the solvent mixture composition is adjusted for optimum operation when dewaxing a given charge stock to one pour point, then that solvent mixture contains too high a proportion of anti-solvent to be tolerated in dewaxing the same charge stock to a lower pour point without separation of a separate oil phase. On the other hand, if the solvent blend is adjusted for optimum operation when dewaxing a given charge stock to a lower pour point, then that solvent mixture will require an excessive temperature differential when the oil is dewaxed to a higher pour point. Therefore, when the operating conditions are so changed to yield a product of a different pour point but with the same charge stock, it is also desirable, or sometimes even necessary, to change the composition of the solvent mixture.

It is a principal object of the present invention to provide an improved method of sequential solvent dewaxing of different waxy lubricating oil fractions of different boiling ranges, such as light, medium and heavy distillate lubricating oil fractions and residual or bright stock lubricating oil material. Another object is to provide an improved method of dewaxing a lubricating oil fraction of either a distillate or residual type to alternately different pour points. It is a further object to provide an improved method of alternately solvent-dewaxing a raflinate lubricating oil fraction from solvent extraction of a lubricating oil distillate fraction and a raffinate lubricating oil fraction from solvent extraction of a deasphaltized short residue from vacuum distillation of lubricating oil distillates from a waxy lubricating oil crude or topped crude stock.

Another object of the invention is to provide an improved.

3 to the accompanying drawing, wherein the sole figure is a schematicprocess flow diagram embodying the invention.

In most continuous commercial scale solvent dewaxing processes, the recovery of the solvent mixture from the oil solvent mixture solution, after removal or the precipitated wax, is effected by heating" the solution and vaporizing the solvent mixture, thus separating the more volatile solvent mixture from the less volatile oil. The dewaxed oil product is recovered as bottoms from the vaporization operation and the solvent mixture is recovered by condensing the overhead vapors. The condensed solvent mixture is then recycled back to the incoming wavy oil charge, thus completing the solvent mixture circuit. It is well known that, in sucha solvent recovery process, it is often more economical of heat requirements to eliect the vaporization in two or more steps, in which the solvent is flashed from the filtrate at successively higher pressures and/or temperatures. In the usual case the solvent and anti-solvent will have different volatilities or in the cases of multicomponent solvent and/ or multicomponent anti-solvents, the average solvent volatility will be different from the average antisolvent volatility. Therefore, the vaporized solvent mixture from certain of the vaporization steps is richer in the anti-solvent than is the bulk of the circulating solvent mixture, and conversely, the vaporized solvent mixture from the rest of the vaporization steps is poorer in antisolvent than is the bulk of the circulating solvent mixture.

It has now been found that the desired change of composition of the solvent mixture can be accomplished during the uninterrupted operation of the process by a particular manipulation of the separate streams of recovered solvent mixture containing different concentrations of solvent and anti-solvent. In accordance with the present invention the composition of the circulating solvent mixture is adjusted at appropriate points of the sequential cycle of operations by diverting to temporary storage certain streams of the recovered solution mixture from the vaporization steps.

The present invention is useful where the optimum composition of the circulating solvent mixture is sub stantially different at different parts of the operating cycle, whether because heavier and lighter wa'xy fractions are alternately charged during the cycle,- 01 because charge stock of the same molecular weight is alternately dewaxed at different operating conditions (dewaxing temperatures) in order to produce dewaxed products of different pour points. It is advantageous, for example, where the concentration of anti-solvent in the solvent mixture is as little as 1 to 4% by volume higher during one part of theoperating cycle than during another part. It is particularly useful, however, where the optimum anti-solvent concentration varies to a greater extent, for example, to by volume or the solvent mixture.

The sequential cyclic operation of the resent invention will he most readily understood by the division of acon-h pl'cte cycle into four phases, asfollows:

Phase 1 is the operation during the charging of arelalively light'lubricating oil fraction, or during the down ing of a particular charge stock to a relatively high pour point, wherein the composition of the circulating solvent mixture remains constant at a relatively high concentration of anti-solvent.

Phase II is the operation near the end of the charging of the relatively light fraction, or the devvaxing to a rela tively' high pour point, wherein the composition of the solution mixture is being changed to a lower anti-solvent concentration in preparation for the changing of the waxy oil charge from a lighter to a heavier fraction or of the product pourpoint from a relatively highto a relativelyjlow pour point.

.Ph'aseIIl is the, operation during the entire period of charging of a relatively heavy fraction, or of producing a relatively low pour point product, wherein the com position of the circulating solvent mixture remains constant at a relatively low concentration of anti-solvent.

Phase IV is the operation beginning with the switch from a relatively heavy fraction charge to a relatively light fraction charge, or with the switch from low to high pour point product and wherein the composition of the circulating solvent mixture is being changed to the relatively high anti-solvent concentration desired for optimum dewaxing under these conditions. Phase I begins immediately at the end of phase IV to complete the cycle.

The drawing, comprising a single figure, is a schematic flow diagram, by reference to which the manipulation of the solvent mixture streams in accordance with the present invention will be explained.

The flow of waxy oil charge to the process and that of the solvent mixture to be admixed with it are continuous' throughout the cycle. The charge enters through line 1 being either distillate or residue, and is mixed with solvent mixture from line 2. The resulting solution enters the refrigeration and wax separating zone 3, where the wax is precipitated from the solution and is separated by any of a number of well known techniques, for example, filtration, centrifugation or settling. The separatedwax product is removed through line 4, and where desired, further purified. The oil-solvent mixture solution is transferred through line '7 from zone 3 to a first flash zone 5, wherein part of the solvent mixture is separated from the solution by vaporization and, after condensation, is removed through line 8. The rest of the solution is transferred through line 9 to a second flash zone 6 wherein the remainder of the solvent mixture is separated from the solution by vaporization and, aftercondensation, is removed through line 11. The liquid residue from this last flash zone is the dewaxed oil product which is removed through line 10 and may be further purified or treated as desired.

During phases I and III, wherein the composition of the solvent mixture is held constant, the two condensed solvent mixture streams from the flash Zones 5 and 6 are combined and the total stream is recycled back to be mixed with a further portion of the entering waxy oil charge, thus completing the solvent mixture circuit. During one of these phases (I or III) the combined stream flows through line 12 to the A storage zone 13, and directly out again, at the same rate, through line 14 to line 2. During the other of these phases, the combined stream flows through line 22 to the B storage zone 20 and directly out again, at substantially the same rate, through line 21- to line 2. Alternately during phases I and III, the solvent mixture may by-pass the storage zones through appropriate by-pass lines which are not shown. The solvent mixture thus traverses a closed continuous circuit at a substantially constant rate and a substantially constant composition during each of phases I and 111,. any unavoidable loss of solvent mixture or components thereof being made-up as required.

However, because of the difference in average volatil itiesof the solvent and the anti-solvent, the composition of the mixture from the first flash zone 5 (as compared withthe composition of the solvent mixture entering line 2-) is always richer in anti-solvent if the average volatility of the anti-solvent is higher than that of the solvent, or always: richer in the solvent if the average volatility of the solvent is higher than that of the anti-solvent; and, accordingly, the composition of the mixture from the second flash zone 6 always differs in the opposite direction.

Where the anti-solvent is more volatile than the solvent, in accordance with the present invention, the s0lvent mixture is circulated through A storage zone 13 during phase- I and through B storage zone 20 during phase III; and conversely, where the anti-solvent: is less light (e. g., distillate) charge and a heavy (e. g., residual) charge, during phase I the waxy oil charge is, say, a distillate material and the total circulating solvent mixture flows through line 12 to A storage zone 13 and out through line 14 back to the entrance of the waxy oil charge. When it is desired to adjust the composition of the circulating solvent mixture to a lower anti-solvent concentration in preparation for switching to, say, a residual charge, phase H is commenced by opening valves 18 and 19 and closing valves 15 and 17. The total circulation rate of the solvent mixture through line 2 is not changed. Thus, the inventory in A storage 13 is increased and the inventory in B storage zone is decreased. The material diverted from the circulating solvent mixture to A storage zone 13, being from only the first flash zone 5, contains a higher proportion of the more volatile anti-solvent than does the solvent mixture enter ing line 2. At the same time the material which is circu lated to B storage zone 20, being from only the second flash zone 6, contains a lower proportion of the more volatile anti-solvent than does the solvent mixture entering line 2. Therefore, during phase II the circulating solvent mixture is constantly depleted of the anti-solvent and the composition of the circulating solvent mixture desired for phase III is quickly reached. At this point phase III is commenced by opening valve 15 and closing valve 16 thus efiecting total solvent mixture circulation from both flash zones through B storage zone 20 The inventory of high anti-solvent concentration solvent mixture in A storage zone 13 remains dormant throughout this phase.

When the desired quantity of residual charge has been processed, phase III is completed. Phase IV commences with the switch to distillate charge stock. At this point, in order to raise the anti-solvent content of the circulating solvent mixture, valves 16 and 17 are opened and valves 15 and 18 are closed. Here, too, the total circulation rate of the solvent mixture through line 2 remains constant and therefore the inventory in B storage zone 20 is increased and the inventory in A storage zone 13 is decreased. The material diverted from the circulating solvent mixture to B storage zone 20, being from only the second flash zone 6, contains a lower proportion of the more volatile anti-solvent than does the solvent mixture entering line 2. At the same time, the material which is circulated to A storage zone 13, being from only the first flash zone 5, contains a higher proportion of the more volatile anti-solvent than does the solvent mixture entering line 2. Therefore, during phase IV the circulating solvent mixture is constantly enriched in anti-solvent and the composition of the circulating solvent mixture desired for phase I is quickly reached. At this point phase I is commenced by opening valve 15 and closing valve 19, thus again eflecting total solvent mixture circulation from both flash zones through A storage zone 13. The inventory of low anti-solvent concentration solvent mixture in B storage zone 20 remains dormant throughout phase I. Thus, the full cycle of operations is completed.

When the cycle of operations consists in dewaxing the same charge stock to alternately difierent pour points, the manipulation of the various solvent mixture streams will be exactly the same as the foregoing case, the only diflerence in the process being that the duration of the phases will be defined, as indicated above, in terms of 6 the changes in product pour point instead of in terms of the changing of charge stocks.

Where there are more than two vaporization steps in the solvent mixture recovery, it will be readily apparent that the various condensed solvent mixture streams may be combined in such a manner as to provide two solvent mixture streams, one of which contains a higher concentration of anti-solvent than does the circulating solvent mixture and the other of which contains a lower concentration of anti-solvent than does the circulating solvent mixture. After such combining of said streams, the process is effected in exactly the same manner as described above, one of the streams flowing throug'h'line 8 and the other flowing through line 11.

The invention will be more readily understood from a description of a specific application thereof. The circulating solvent mixture is a mixture of methyl ethyl ketone (boiling point, about C.) as the anti-solvent and toluene (boiling point, about 111 C.) as the solvent. The charge stocks are an East Texas solvent-extracted lubricating oil distillate (viscosity of 250 SSU at F.) and a bright stock raflinate obtained by deasphalting and solvent-extracting an East Texas Short Residue. Both charge stocks are dewaxed to a pour point of +5 F. The concentration of methyl ethyl ketone is adjusted to 65% by volume during phase IV for the dewaxing of the distillate during phase I and to 48% by volume during phase II for the dewaxing of the bright stock during phase III. The flow of solvent mixture from the storage zones is held constant at about 1,050 gallons per minute throughout the cycle. The flow of solvent mixture from the first flash zone is 500 gallons per minute and the concentration of methyl ethyl ketone in the condensed solvent mixture from the first flash zone is, in this specific example, about 8.3% by volume higher than the concentration of the circulating solvent mixture during phase I and slightly more than 8.3% by volume higher during phase III, with intermediate values during phases II and IV. The flow of solvent mixture from the other (second) flash zone is 550 gallons per minute and its methyl ethyl ketone content is accordingly about 7.5% by volume less than that of the circulating solvent mixture during phase I and slightly more than 7.5 by volume less during phase III, with intermediate values during phases II and IV. (Although the magnitude of the concentration difference between the solvent mixture streams from the flash zones will vary slightly depending upon the composition of the circulating solvent mixture entering the flash zones, this difference will, nevertheless, be in the same direction for a given combination of solvent and anti-solvent volatilities.) A storage contains a working inventory of 700 barrels of solvent mixture during phase I. The inventory increases gradually to 2,000 barrels during phase II and is dormant at that figure during phase III. Then, during phase IV the inventory is reduced gradually to 700 barrels. B storage" contains a dormant inventory of 2,000 barrels of solvent mixture during phase I, the inventory gradually decreasing to 700 barrels during phase II, remaining at a working level of 700 barrels through phase III, and gradually increasing again to 2,000 barrels during phase IV. Thus, it will be seen that phases II and IV are each completed in about three hours. Since the average length of the complete operating cycle in the usual commercial plant is 25 to 30 days, it is evident that the three hour changeover from one solution mixture composition to another is a negligible part of the cycle time. Therefore, both types of charge stock are almost entirely dewaxed at as close an approach to optimum conditions as possible and with no interruption of the process.

When it is desired to dewax the oil fractions to a still lower pour point, e. g., 0 F., or 5 F., or 10 F., etc., it will be understood that the proportion of solvent in the solvent mixture will be higher than when dewaxing-each of the oil fractions as in the foregoing example. Thus for 7 dewaxing the bright stock the composition oi the mixture may be 45% by volume methyl ethyl ketone and 55% by volume toluene, or even 42% by volume methyl ethyl ketone and 58% by volume toluene, and correspond ingly higher proportions of solvent may be used in dewaxing the distillate fraction. Conversely when the oil fractions are to be dewaxed to a higher pour point than +5 Fa, it will be possible to use lower proportions of the solvent for both distillate and bright stock-fractions.

The invention is applicable to many dewaxing solution mixtures as long as the solvent proportion of the mixture has a substantially different average volatility than does the anti-solvent proportion; For example, benzene (solvent) and furfural (anti-solvent); benzene (solvent) and acetone (anti-solvent); toluene and benzene (solvent) and methyl ethyl ketone (anti-solvent); toluene (solvent) and methyl isobutyl ketone (anti-solvent); sulfur dioxide (solvent) and dichlorethylene (anti-solvent); and many others.

We claim as our invention:

1. In a continuous method for sequential blocked-out cyclic solvent dewaxing of waxy distillate lubricating oil fractions and waxy residual lubricating oil fractions wherein a circulating solvent mixture used is a mixture of a solvent and an anti-solvent having dilferent average volatilities and whereinrecovery of the solvent mixture from dewaxed lubricating oil-solvent mixture solution from which precipitated wax has been removed is efiected in a plural number of sequential vaporization steps, the improvement which comprises increasing the anti-solvent concentration of the circulating solvent mixture at the beginning of charging of waxy distillate stock by diver'ting to a first storage zone solvent mixture from a number of said vaporization Steps which yield a solvent mixture stream containing a concentration of anti-solvent lower than the concentration of the antisolvent in the solvent mixture portion of said solution while maintaining the circulation rate of solvent mixture by adding solvent mixture from inventory in a second storage zone until a desired higher concentrationof the anti-solvent in the circulating solvent mixture has been attained, at which time diverting the subsequent stream of lower anti-solvent concentration from said number of vaporization steps back to the circulating solvent mixture, and toward the end of said charging of waxy distillate stock decreasing the anti-solvent concentration of the circulating solvent mixture by diverting to said second storage vessel solvent mixture from others of said vaporization steps whichyield a solvent mixture stream containing a concentration of anti-solvent higher than the concentration of the anti-solvent mixture portionof said solutionwhile maintaining the circulation rate of solvent mixture by adding solvent mixture from inventory in said first storage zone until a desired lower concentration of the anti-solvent in the circulating solvent mixture has been attained at which time diverting the subsequent stream of higher anti-solvent concentration from said other number of vaporization steps back to the circulating solvent mixture and at which time commencing charging of residual waxy stock. 2. The process according to claim 1 wherein the total number of sequential vaporization steps is two. 3t The process according to claim 1 wherein the solvent mixture comprises methyl ethyl ketone as antisolvent and toluene as solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,061,541 Govers Nov. 17, 1936 1,397,868 Jenkins Apr. 2, 194d 2,463,845 Eacklund et al. Mar. 8, I949 2,565,48 Fischer Aug. 28, 195i 

1. IN A CONTINUOUS METHOD FOR SEQUENTIAL BLOCKED-OUT CYCLIC SOLVENT DEWAXING OF WAXY DISTILLATE LUBRICATING OIL FRACTIONS AND WAXY RESIDUAL LUBRICATING OIL FRACTIONS WHEREIN A CIRCULATING SOLVENT MIXTURE USED IS A MIXTURE OF A SOLVENT AND AN ANTI-SOLVENT HAVING DIFFERENT AVERAGE VOLATILITIES AND WHEREIN RECOVERY OF THE SOLVENT MIXTURE FROM DEWAXED LUBRICATING OIL-SOLVENT MIXTURE SOLUTION FROM WHICH PRECIPITATED WAX HAS BEEN REMOVED IS EFFECTED IN A PLURAL NUMBER OF SEQUENTIAL VAPORIZATION STEPS, THE IMPROVEMENT WHICH COMPRISES INCREASING THE ANTI-SOLVENT CONCENTRATION OF THE CIRCULATING SOLVENT MIXTURE AT THE BEGINNING OF CHARGING OF WAXY DISTILLATE STOCK BY DIVERTING TO A FIRST STORAGE ZONE SOLVENT MIXTURE FROM A NUMBER OF SAID VAPORIZATION STEPS WHICH YIELD A 